I/O subsystem architecture

The primary concerns of I/O subsystem architecture include the coordination of central processor and I/O subsystem activities, the mapping of external I/O processing requirements into the basic functionality of the I/O subsystem, and the management of concurrent processing activities within the I/O subsystem itself. Architectural developments such as data and command chaining, channel program interfaces, automatic key searching, rotational portion sensing, and block multiplexing are evaluated in terms of these underlying concerns. A number of new arguments are presented to demonstrate the strengths and weaknesses of current designs, and several projections are made concerning the future evolution of this area.